1. Field
This invention relates generally to ultrasonic inspection of welds and more particularly, to apparatus and methods utilized for the ultrasonic inspection of welds on access hole covers found in boiling water reactors containing jet pumps.
2. Description of Related Art
In general, a BWR produces electrical power by heating water to its boiling temperature in a reactor vessel that contains a nuclear fuel core in order to generate steam which is used in turn to drive a steam turbine. The nuclear fuel core consists of a plurality of fuel bundle assemblies. As shown in FIG. 1, in a BWR, feedwater enters a reactor vessel 110 via a feedwater inlet 112. The feedwater flows downwardly through a downcomer annulus 116 to a core lower plenum 124. The downcomer annulus 116 is an annular region located between the reactor vessel and a core shroud 118. The feedwater then enters a nuclear fuel core 120 which includes a plurality of fuel assemblies 122. The core shroud 118 is a stainless steel cylinder which surrounds the nuclear fuel core 120. A mixture of water and steam enters a core upper plenum 126 under the shroud head 128. Jet pump assemblies 142 are circumferentially distributed around the core shroud 118. This core shroud 118 is supported by a shroud support 51 and a shroud support plate 152 is welded to the shroud support 151.
During the construction of some BWRs, access holes were cut into the core shroud support plate to provide personnel a means of passage into the core lower plenum. Upon completion of construction, these holes were covered by welding a plate in order to ensure a leak-tight barrier between the annulus and the lower plenum. Inspection of these welds is conducted on a routine basis to assure that they remain leak tight and to identify any potential concerns for defects. The inspection is typically carried out using conventional ultrasonic inspection techniques known in the art.
Non-destructive evaluation (NDE) methods, such as ultrasonic testing (UT), are known in the art and are typically employed to inspect a structure for defects. In general, high frequency sound waves are applied to the structure being tested using one or more transducers. The transducers typically comprise piezocrystal elements that are excited by an electrical voltage in order to induce the ultrasonic waves in the structure. When the sound waves interact with something (e.g., a void; a crack or other defect) having a significant difference in impedance from that of the propagation medium, a portion of the sound is either reflected or diffracted back to the source from which it originated. Detection and quantification of the returned sound pattern is used to determine the characteristics of the reflecting medium. The results obtained from the inspection are utilized to assess the condition and integrity of the structure. Assessment of the structure is based on the characteristics of the detected defects, such as, for example, the size, orientation and location of the defects. The more precise and accurate the inspection technique and data obtained, the more reliable is the assessment for determining the condition of the structure.
It is an object of the invention to provide apparatus and methods for the ultrasonic inspection of the welds on access hole covers found in boiling water reactors, e.g., the shroud support plate, having jet pumps to detect defects in the welds in order to assure that defects are identified and resolved such that a serious situation can be prevented.